In particular in institutional washing machines, including institutional laundry and in particular dishwashing machines, a product to be dispensed, e.g. a detergent, a conditioner, a rinse aid and the like, no unit dosages of said products are used. Rather single doses are obtained by dispensing a certain amount from a stock of said product contained in a reservoir inside the washing machine. Thus, in institutional washing machines, in particular in institutional dishwashing machines, there is a need to automatically control the dosing of these products into said washing machines from the reservoir which is connected to the rest of the washing machine, in particular the wash tank, by a reversibly closable output device, usually a valve. In institutional dishwashing machines usually large blocks or “bricks” of solid detergents, comprising a large number of single doses, are placed in such a reservoir and then are sprayed with water or diluted washing liquor from a spray nozzle to dissolve some of the detergent. To control the desired product concentration a dispenser controller usually is used in such washing machines controlling the product concentration in the washing machine by controlling dispensing of the product. Commonly, a sensor is located for example in the wash tank of such a washing machine measuring a parameter corresponding to the concentration of the product in the washing liquor present in said wash tank, which is coupled to the controller.
As already described in U.S. Pat. No. 5,500,050 such systems often suffer from the problem of controlling the product concentration closely about the desired setpoint with little over- or undershoot. Such under- or overshootings occur for example if a well soluble product is used (e.g. having a solubility in water having a temperature of 20° C. equal to or above 1 g/L, preferably of equal to or above 5 g/L), if the distance between the outlet of the product reservoir (the dosing point) and the sensor is rather large, as it is the case in many commercially available institutional single tank dishwashing machines or due to the decrease in feed rate over the lifetime of the product block or brick because of its decreasing size which leads to a larger distance between the spray nozzle and the block or brick. The dissolution and mixing time of the product in the washing liquor further is influenced by the temperature of both, the spray water and the washing liquor, the pressure at the spray nozzle, the intensity of mixing in the wash tank, the composition of the product and the like. It also should be borne in mind that a considerable amount of the product still maybe in the feed line connecting the dispenser to the wash tank when measuring the concentration in the wash tank.
Conventional washing machines use a simple control function which initiates dispensing of the product to the machine once the concentration in the wash tank drops below a given setpoint and do not stop dispensing until the sensor measures reaching of the setpoint. In consequence, the final concentration after dispensing typically is 50% or even more above the setpoint. This is undesirable from both, an economic as well as an ecologic point of view. In addition, due to the highly alkaline pH of detergents for institutional dishwashing machines, a constant overdosing also may result in severe glass corrosion. Too low a detergent concentration on the other hand leads to a poor cleaning result.
To eliminate at least some of these drawbacks, U.S. Pat. No. 5,500,050 describes a detergent dispenser controller which determines the detergent concentration in a dishwasher's water tank by measuring the conductivity therein and automatically learns the current feed rate of the detergent dispenser based on a moving average of then last feed cycles. In this way, large over- and undershootings due to the decrease of detergent block over time, for instance, may be minimized.
However, even using the method described in U.S. Pat. No. 5,500,050 over- and undershooting of product concentration still may be observed to an unfavourable extent. It was therefore an object of the present invention to provide a method of controlling a dispenser for dosing a product in a washing machine which allows to closely control the concentration of the product in a washing machine, but does not require any structural alterations with respect to mechanical parts of said washing machine.
This object is solved by the method of the present invention.